A very broad question; general common signs of wear are varied & may included any of the following &/or combinations thereof (apart from the "no go/difficulty starting/rust" scenario's):-
1. Engine
- Excessive smoking &/or overheating
- Lack of power
- Excessive oil consumption &/or oil "interior" contamination of the air filter &/or "blow back/fuming" from the dipstick tube
- Oil contamination (via water &/or fuel &/or metal)
- Miss firing
2. Transmission
- Excessive vibration/harsh engagement
- Lack of power/torque &/or difficulty of engagement
- When warm slippage under load &/or fwd/rev direction changes
- Excessive heat
- Oil contamination/discolouration &/or premature belt/cone wear
3. Deck/Axles/Frame
- Unusual vibration &/or high pitched "squealing" evident (indicating spindle/pulley/bearing/belt wear..etc)
- Uneven/irregular cutting&/or deck unable to maintain consistent cutting height
- Signs of oil/grease "bleeding" from components
- Premature belt wear/throwing of belts
- Excessive play in steering linkage
4. Electrics
- Blown fuses/frayed or degrading wiring
- Continually blowing light globes
- Charging system not supplying sufficient charge (to battery..etc)
- Starter spinning slowly/harsh engagement